A London listing for shares in Saudi Aramco, the world's biggest oil firm, would be "inappropriate" if it meant bending the rules, according to a pension firm.

Saudi Arabia is said to want to sell a 5% stake in the company as it plans to expand its economy away from oil.

Aramco is thought to be considering selling shares in London or New York, among other markets.

However, London listing rules suggest more than 25% of shares should be sold.

Regulators want to ensure enough shares are available to be bought and sold to make a functioning market. But exceptions can be made for companies who can show that will still be possible while selling a smaller stake in themselves.

'Money to be made'

But Ashley Hamilton Claxton, corporate governance manager at Royal London Asset Management, said the rules should not be bent.

"It would be highly inappropriate for us to be bending listing rules and bending benchmark rules to accommodate this one large company," she told the BBC's Today programme.

"There's lots of money to be made for investment bankers and advisers, but very little protection for pension savers and that's something we really need to keep at the heart of this situation."

Her view echoes that of the Investment Association, which has said there should not be special cases for share listings in London.

Tech valuation

The company may attract a valuation of as much as $2tn, according to some estimates.

But Stephen Clapham, of consultancy Behind the Balance Sheet, said that price "doesn't make any sense to me".

Financial data for the company is hard to come by, he told Today, but he estimates Aramco's annual turnover is about $185bn. A $2tn price tag would suggest it is worth more than 10 times its sales, a scale of valuation normally reserved for technology companies that are reckoned to be in a growth spurt.

Mr Clapham said Aramco could duck the 25% rule by placing the stake in a separate holding company and selling all that company's stock in London. However, he added that a holding company like that would probably need to invest in other companies too.